A milkman has a can of 20 gallons, full of milk. But due to a sudden epidemic, all his cows in the farm fall sick. He can no longer collect milk from them.

To conserve his milk he decides that on the first day he will sell one liter and refill the can back with water. On the 2nd day he will take out 2 liters and refill the bottle. On the 3rd day he will take out 3liters and so on...

By the time all the milk is gone, how much water has the milkman sold?